Implement LWG 2451, optional<T> should 'forward' T's implicit conversions.
Implement LWG 2451, optional<T> should 'forward' T's
implicit conversions.
* include/experimental/optional (__is_optional_impl, __is_optional):
New.
(optional()): Make constexpr and default.
(optional(_Up&&), optional(const optional<_Up>&),
optional(optional<_Up>&& __t): New.
(operator=(_Up&&)): Constrain.
(operator=(const optional<_Up>&), operator=(optional<_Up>&&)): New.
* testsuite/experimental/optional/cons/value.cc:
Add tests for the functionality added by LWG 2451.
* testsuite/experimental/optional/cons/value_neg.cc: New.
From-SVN: r238049
6ffe854859